Method of Forming a Metal Contact Opening with a Width that is Smaller than the Minimum Feature Size of a Photolithographically-Defined Opening

ABSTRACT

The width of a metal contact opening is formed to be smaller than the minimum feature size of a photolithographically-defined opening. The method forms the metal contact opening by first etching the fourth layer of a multilayered hard mask structure to have a number of trenches that expose the third layer of the multilayered hard mask structure. Following this, the third, second, and first layers of the multilayered hard mask structure are selectively etched to expose uncovered regions on the top surface of an isolation layer that touches and lies over a source region and a drain region. The uncovered regions on the top surface of the isolation layer are then etched to form the metal contact openings.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of forming a metal contactopening and, more particularly, to a method of forming a metal contactopening with a width that is smaller than the minimum feature size of aphotolithographically-defined opening.

2. Description of the Related Art

A metal oxide semiconductor (MOS) transistor is a well-knownsemiconductor device which can be implemented as either an n-channel(NMOS) device or a p-channel (PMOS) device. A MOS transistor hasspaced-apart source and drain regions, which are separated by a channel,and a gate that lies over the channel. The gate is insulated from thechannel by a gate dielectric layer. A metal-gate MOS transistor is atype of MOS transistor that utilizes a metal gate and a high-k gatedielectric layer.

Metal-gate MOS transistors are connected to a metal interconnectstructure that electrically connects the MOS transistors together toform an electrical circuit. The metal interconnect structure includeslayers of metal traces that are electrically isolated from each other bylayers of isolation material, and metal vias that extend through thelayers of isolation material to electrically connect adjacent layers ofmetal traces.

The metal interconnect structure also includes metal contacts thatextend through the bottom layer of isolation material to make electricalconnections to the source and drain regions of the MOS transistors. Themetal contacts are formed in metal contact openings that extend throughthe bottom layer of isolation material to expose the source and drainregions.

Conventionally, the metal contact openings are fabricated by forming apattered photoresist layer on the bottom isolation layer, which touchesand lies over the source and drain regions. Once the patternedphotoresist layer has been formed, the bottom isolation layer is etcheduntil the source and drain regions have been exposed.

The etch forms source metal contact openings that expose the sourceregions, and drain metal contact openings that expose the drain regions.The patterned photoresist layer is then removed. After this, silicidelayers are formed on the source and drain regions, followed by theformation of metal contacts that lie in the source and drain metalcontact openings, and touch the source and drain silicide layers and thebottom isolation layer.

Thus, in the conventional approach, the widths of the source and drainmetal contact openings are determined by the widths of the openings inthe patterned photoresist layer. As a result, the minimum widths of thesource and drain metal contact openings are determined by the minimumfeature size that can be photolithographically printed with adequatecontrol.

The minimum feature size has two basic limits: the smallest image thatcan be projected onto a wafer, and the resolving capability of thephotoresist to make use of that image. The smallest image that can beprojected onto a wafer is determined by the wavelength of the imaginglight and the numerical aperture of the projection lens. The resolvingcapability of the photoresist is determined, in part, by the shape ofthe image projected onto the wafer.

For example, when long parallel lines are projected onto the wafer, thephotoresist has a higher resolving capability along the lengthwise edgesof the lines than when square or circular shapes are projected onto thewafer. As a result, long parallel lines can be formed with smallerminimum feature sizes than square or circular openings.

To increase the density of devices formed on the wafer and therebyreduce costs, the minimum feature size has been continuously scaleddown, primarily by decreasing the wavelength of the imaging light andincreasing the numerical aperture. However, the density of devicesformed on the wafer can be further increased if the metal contactopenings could be formed to have widths that are smaller than theminimum feature size of a photolithographically-defined opening. Thus,there is a need for a method of forming a metal contact opening with awidth that is smaller than the minimum feature size of aphotolithographically-defined opening.

SUMMARY OF THE INVENTION

The present invention provides a method of forming a semiconductorstructure that increases the density of the devices formed on a wafer.The method includes forming a first hard mask layer that touches andlies over an isolation layer. The isolation layer has a top surface, andtouches and lies over a source structure and a drain structure. Themethod also includes forming a second hard mask layer that touches andlies over the first hard mask layer. The second hard mask layer has atop surface and a bottom surface. The method additionally includesforming a third hard mask layer that touches and lies over the secondhard mask layer. The third hard mask layer has a top surface. The methodfurther includes forming a fourth hard mask layer that touches and liesover the third hard mask layer. The fourth hard mask layer has a topsurface. In addition, the method includes etching the fourth hard masklayer to form a number of trenches. Each trench exposes the top surfaceof the third hard mask layer.

The method of the present invention alternately includes forming a firsthard mask layer that touches and lies over an isolation layer. Theisolation layer has a top surface, and touches and lies over a sourcestructure and a drain structure. The method also includes forming asecond hard mask layer that touches and lies over the first hard masklayer. The second hard mask structure has a top surface and a bottomsurface. In addition, the method includes forming a third hard masklayer that touches and lies over the second hard mask layer. The thirdhard mask layer has a top surface. Further, the method includes forminga fourth hard mask layer that touches and lies over the third hard masklayer. The fourth hard mask layer has a top surface. The methodadditionally includes forming a first patterned photoresist layer thattouches and lies over the fourth hard mask layer. The method furtherincludes forming a second patterned photoresist layer that touches andlies over the fourth hard mask layer. The first patterned photoresistlayer and the second patterned photoresist layer expose a number ofuncovered regions on the top surface of the fourth hard mask layer.

A better understanding of the features and advantages of the presentinvention will be obtained by reference to the following detaileddescription and accompanying drawings which set forth an illustrativeembodiment in which the principals of the invention are utilized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D through 9A-9D are views illustrating an example of a method100 of forming a metal contact opening in accordance with the presentinvention. FIGS. 1A-9A are plan views. FIGS. 1B-9B are cross-sectionalviews taken along lines 1B-1B through 9B-9B of FIGS. 1A-9A. FIGS. 1C-9Care cross-sectional views taken along lines 1C-1C through 9C-9C of FIGS.1A-9A. FIGS. 1D-9D are cross-sectional views taken along lines 1D-1Dthrough 9D-9D of FIGS. 1A-9A.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A-1D through 9A-9D show views that illustrate an example of amethod 100 of forming a metal contact opening in accordance with thepresent invention. FIGS. 1A-9A show plan views, while FIGS. 1B-9B showcross-sectional views taken along lines 1B-1B through 9B-9B of FIGS.1A-9A, FIGS. 1C-9C show cross-sectional views taken along lines 1C-1Cthrough 9C-9C of FIGS. 1A-9A, and FIGS. 1D-9D show cross-sectional viewstaken along lines 1D-1D through 9D-9D of FIGS. 1A-9A.

As shown in FIGS. 1A-1D, method 100 utilizes a conventionally-formedmetal-gate MOS transistor structure 108. MOS transistor structure 108,in turn, includes a semiconductor body 110 that has asingle-crystal-silicon substrate region 112, and a trench isolationstructure 114 that touches substrate region 112.

In addition, semiconductor body 110 includes a source 120 and a drain122 that each touch substrate region 112. The source 120 and drain 122each has a conductivity type that is the opposite of the conductivitytype of substrate region 112. Source 120 includes a lightly-doped region120L, and a heavily-doped region 120H. Similarly, drain 122 includes alightly-doped region 122L, and a heavily-doped region 122H. Further,substrate region 112 has a channel region 124 that lies between source120 and drain 122.

As also shown in FIGS. 1A-1D, MOS transistor structure 108 includes ahigh-k gate dielectric structure 126 that touches and lies over channelregion 124, and a metal gate 130 that touches gate dielectric structure126 and lies over channel region 124. In addition, MOS transistorstructure 108 includes a sidewall spacer 132 that laterally surroundsgate 130, and a bottom isolation layer 138 that touches sidewall spacer132. Bottom isolation layer 138 also touches and lies over source region120H and drain region 122H.

As further shown in FIGS. 1A-1D, method 100 begins by forming a firsthard mask layer 150 that touches and lies over bottom isolation layer138. The first hard mask layer 150 can be implemented with, for example,a layer of silicon oxynitride (SiON) or a layer of silicon carbonnitride (SiCN).

After the first hard mask layer 150 has been formed, a second hard masklayer 152 is formed to touch and lie over the first hard mask layer 150.The second hard mask layer 152, which is substantially thicker than thefirst hard mask layer 150, can be implemented with, for example, achemically-vapor deposited (CVD) layer of an amorphous carbon materialsuch as an advanced patterning film (APF).

After the second hard mask layer 150 has been formed, a third hard masklayer 154 is formed to touch and lie over the second hard mask layer152. The third hard mask layer 154 can be implemented with, for example,a layer of silicon nitride (SiN) or a layer of silicon oxynitride(SiON).

After the third hard mask layer 154 has been formed, a fourth hard masklayer 156 is formed to touch and lie over the third hard mask layer 150.The fourth hard mask layer 156 can be implemented with, for example, alayer of oxide that touches and lies over the third hard mask layer 154,and a layer of silicon nitride (SiN) that touches and lies over thelayer of oxide. The first hard mask layer 150 is thicker than thecombined thicknesses of the third hard mask layer 154 and the fourthhard mask layer 156.

After the fourth hard mask layer 156 has been formed, a patternedphotoresist layer is formed is formed as a number of spaced-apart strips160 that touch and lie over the fourth hard mask layer 156. The strips160 of patterned photoresist layer are formed in a conventional manner,which includes depositing a layer of photoresist, projecting a lightthrough a patterned black/clear glass plate known as a mask to form apatterned image on the layer of photoresist, and removing the imagedphotoresist regions, which were softened by exposure to the light. Thestrips 160 of patterned photoresist layer can also include an underlyinganti-reflective coating.

Pitch is the distance from one edge of a feature to a corresponding edgeof an adjacent feature. The minimum pitch is equal to 2(Ki)*(λ/NA),where Ki represents the difficulty of the lithographic process (theresolving capability), λ represents the wavelength of the imaging light,and NA represents the numerical aperture of the lens.

Thus, using a current-generation imaging light with a wavelength of 193nm and a lens with a numerical aperture of 1.35 (using water immersion),a minimum pitch of approximately 80 nm can be achieved when the minimumKi approaches its practical limit of approximately 0.28.

In the present example, although a minimum pitch of approximately 80 nmis possible, the strips 160 of patterned photoresist layer are formed tohave a pitch P of 168 nm. Further, each strip 160 has a width W equal to¼ P (42 nm), while adjacent strips 160 are separated by a gap G1 of ¾ P(126 nm).

As shown in FIGS. 2A-2D, after the strips 160 of patterned photoresistlayer have been formed, a second patterned photoresist layer is formedin a conventional manner as a number of spaced-apart stripes 166 thattouch and lie over the fourth hard mask layer 156. The stripes 166 ofpatterned photoresist layer can also include an underlyinganti-reflective coating.

The stripes 166, which are spaced apart from the strips 160, lie betweenthe strips 160 in an alternating manner to expose a number of uncoveredregions on the top surface of the fourth hard mask layer 156. In thepresent example, the stripes 166 of patterned photoresist layer are alsoformed to have the pitch P of 168 nm. Further, each stripe 166 has thewidth W equal to ¼ P (42 nm). Each stripe 166 is also spaced apart fromeach adjacent strip 160 by a gap G2 of ¼ P.

As shown in FIGS. 3A-3D, after the stripes 166 of patterned photoresistlayer have been formed, the uncovered regions on the top surface of thefourth hard mask layer 156 are etched to form a number of trenches 168.Each of the trenches 168 extends through the fourth hard mask layer 156to expose the top surface of the third hard mask layer 154. Followingthis, the strips 160 and stripes 166 of the patterned photoresist layersare removed in a conventional manner, such as with an ash process.

As shown in FIGS. 4A-4D, after the patterned photoresist layers 160 and166 have been removed, a patterned photoresist layer is formed is formedin a conventional manner as a number of spaced-apart strips 170 thattouch and lie over the fourth hard mask layer 156. The strips 170 alsoextend into the trenches 168 to touch and lie over portions of the thirdhard mask layer 154. The strips 170 of patterned photoresist layer canalso include an underlying anti-reflective coating.

The strips 170 are substantially orthogonal to the trenches 168.Further, in the present example, the strips 170 of patterned photoresistlayer are formed to have the pitch P of 168 nm. Further, each strip 170has a width W equal to ¼ P (42 nm), while adjacent strips 170 areseparated by a gap G3 of ¾ P (126 nm).

As shown in FIGS. 5A-5D, after the strips 170 of patterned photoresistlayer have been formed, a patterned photoresist layer is formed in aconventional manner as a number of spaced-apart stripes 176 that touchand lie over the fourth hard mask layer 156. The stripes 176 also extendinto the trenches 168 to touch and lie over portions of the third hardmask layer 154. The stripes 176 of patterned photoresist layer can alsoinclude an underlying anti-reflective coating.

In the present example, the stripes 176 of patterned photoresist layerare also formed to have the pitch P of 168 nm. Further, each stripe 176of patterned photoresist layer has a width W equal to ¼ P (42 nm). Eachstripe 176 is also spaced apart from each adjacent strip 160 by a gap G4of ¼ P.

The stripes 176, which are spaced apart from the strips 170, lie betweenthe strips 170 in an alternating manner. The strips and stripes 170 and176, in combination with the orthogonally-oriented trenches 168, exposea checkerboard pattern of a number of uncovered regions on the topsurface of the third hard mask layer 154. (Regions on the top surface ofthe fourth hard mask layer 156 are also exposed by the strips 170 andstripes 176.)

As shown in FIGS. 6A-6D, after the stripes 176 of patterned photoresistlayer have been formed, the uncovered regions on the top surface of thethird hard mask layer 154 are etched to form a number of third hard maskopenings. The etch continues until a number of uncovered regions on thetop surface of the second hard mask layer 152 are exposed by the thirdhard mask openings. In the present example, the etchant is selective sothat more of the third hard mask layer 154 is etched than the fourthhard mask layer 156.

After the uncovered regions on the top surface of the second hard masklayer 152 have been exposed, the etchant is changed and the uncoveredregions on the top surface of the second hard mask layer 152 are etchedto form a number of second hard mask openings. The etch continues untila number of uncovered regions on the top surface of the first hard masklayer 150 are exposed by the second hard mask openings. In the presentexample, the etchant is selective so that more of the second hard masklayer 152 is etched than the fourth hard mask layer 156 or the thirdhard mask layer 154.

During the etch of the second hard mask layer 152, the strips andstripes 170 and 176 are etched away. In addition, the second hard masklayer 152 is etched with a heavy polymer etch, which forms the secondhard mask openings through the second hard mask layer 152 to havetapered side wall surfaces.

Thus, the thickness of the second hard mask layer 152 determines (alongwith other factors such as the etchant) the widths of the second hardmask openings on the bottom surface of the second hard mask layer 152.In the present example, the second hard mask openings at the top surfaceof the second hard mask layer 152 have widths of approximately 42 nm,while the second hard mask openings at the bottom surface of the secondhard mask layer 152 have widths of approximately 20 nm.

After the uncovered regions on the top surface of the first hard masklayer 150 have been exposed, the etchant is changed and the uncoveredregions on the top surface of the first hard mask layer 150 are etchedto form a number of first hard mask openings. The etch continues until anumber of uncovered regions on the top surface of bottom isolation layer138 are exposed by the first hard mask openings.

During the etch of the first hard mask layer 150, the fourth hard masklayer 156 and the third hard mask layer 154 are removed, therebyexposing the top surface of the second hard mask layer. In the presentexample, the etchant is selective so that more of the first hard masklayer 150 is etched than the second hard mask layer 152. In addition,the first hard mask openings through the first hard mask layer 150 havewidths of approximately 20 nm as a result of the widths of the secondhard mask openings at the bottom surface of the second hard mask 152.

Thus, the uncovered regions of the third hard mask layer 154, theunderlying regions of the second hard mask layer 152, and the underlyingregions of the first hard mask layer 150 are etched to form a number ofmask openings 180 that extend through the second hard mask layer 152 andthe first hard mask layer 150. The mask openings 180 each exposes anuncovered region on the top surface of bottom isolation layer 138.

The mask openings 180 through the second hard mask layer 152 are formedby the second hard mask openings, while the mask openings 180 throughthe first hard mask layer 150 are formed by the first hard maskopenings. Once the mask openings 180 have been formed, the second hardmask layer 152 can optionally be removed. In the present example, method100 continues without removing the second hard mask layer 152 at thispoint.

As shown in FIGS. 7A-7D, after the mask openings 180 have been formed,the etchant is changed and the uncovered regions on the top surface ofbottom isolation layer 138 are etched. The etch forms a number of metalcontact openings 182 in bottom isolation layer 138, where one of themetal contact openings 182 exposes a top surface region of source region120H, and one of the metal contact openings 182 exposes a top surfaceregion of drain region 122H.

In the present example, the metal contact openings 182 each have widthsof approximately 20 nm due to the widths of the first hard maskopenings, which are the same as the minimum widths of the mask openings180. After the metal contact openings 182 have been formed, the firsthard mask layer 150 and the second hard mask layer 152 are removed in aconventional manner.

As shown in FIGS. 8A-8D, once the first hard mask layer 150 and thesecond hard mask layer 152 have been removed, a source metal silicideregion 184 that touches and lies over source region 120H, and a drainmetal silicide region 186 that touches and lies over drain region 122Hare formed in a conventional manner. Following this, a metal contactlayer 188, such as a layer of tungsten (W), is deposited to touch thetop surface of bottom isolation layer 138 and fill up the metal contactopenings 182 in bottom isolation layer 138.

As shown in FIGS. 9A-9D, after metal contact layer 188 has been formed,metal contact layer 188 is planarized in a conventional manner, such aswith chemical-mechanical polishing, to expose the top surface of bottomisolation layer 138. The planarization forms metal contacts 190 in themetal contact openings 182. The metal contacts 190 make electricalconnections to the source and drain metal silicide regions 184 and 186.Method 100 then continues with conventional steps to complete theformation of a metal interconnect structure.

One of the advantages of the present invention is that method 100 formsmetal contact openings 182 which have widths that are substantiallysmaller than the minimum pitch. In the present example, the metalcontact openings 182 have widths of approximately 20 nm, while theminimum pitch P is approximately 80 nm (using a wavelength 193 nm, anumerical aperture of 1.35, and a Ki of 0.28).

Another of the advantages of the present invention is that method 100forms metal contact openings 182 with widths that are substantiallysmaller than the minimum feature size of a photolithographically-definedopening. In the present example, the metal contact openings 182 havewidths of approximately 20 nm, while the minimum feature size of aphotolithographically-defined opening is approximately 90-100 nm whenthe minimum pitch is approximately 80 nm. As a result of forming metalcontact openings 182 which have widths that are substantially smallerthan the minimum pitch and the minimum feature size of aphotolithographically-defined opening, the density of the devices on awafer can be increased substantially.

It should be understood that the above descriptions are examples of thepresent invention, and that various alternatives of the inventiondescribed herein may be employed in practicing the invention. Thus, itis intended that the following claims define the scope of the inventionand that structures and methods within the scope of these claims andtheir equivalents be covered thereby.

What is claimed is:
 1. A method of forming a semiconductor structurecomprising: forming a first hard mask layer that touches and lies overan isolation layer, the isolation layer having a top surface, andtouching and lying over a source structure and a drain structure;forming a second hard mask layer that touches and lies over the firsthard mask layer, the second hard mask layer having a top surface and abottom surface; forming a third hard mask layer that touches and liesover the second hard mask layer, the third hard mask layer having a topsurface; forming a fourth hard mask layer that touches and lies over thethird hard mask layer, the fourth hard mask layer having a top surface;and etching the fourth hard mask layer to form a number of trenches,each trench exposing the top surface of the third hard mask layer. 2.The method of claim 1 and further comprising: forming a first patternedphotoresist layer that touches and lies over the fourth hard mask layer;and forming a second patterned photoresist layer that touches and liesover the fourth hard mask layer, the first patterned photoresist layerand the second patterned photoresist layer exposing a number ofuncovered regions on the top surface of the fourth hard mask layer, thenumber of uncovered regions on the top surface of the fourth hard masklayer being etched to form the number of trenches.
 3. The method ofclaim 1 and further comprising selectively etching the third hard masklayer, the second hard mask layer, and the first hard mask layer to forma number of mask openings, each mask opening exposing an uncoveredregion on the top surface of the isolation layer.
 4. The method of claim3 and further comprising: forming a third patterned photoresist layerthat touches and lies over the third hard mask layer and the fourth hardmask layer; and forming a fourth patterned photoresist layer thattouches and lies over the third hard mask layer and the fourth hard masklayer, the third patterned photoresist layer and the fourth patternedphotoresist layer exposing a number of uncovered regions on the topsurface of the third hard mask layer, the number of uncovered regions onthe top surface of the third hard mask layer being etched to form thenumber of mask openings.
 5. The method of claim 3 and further comprisingetching each uncovered region on the top surface of the isolation layerto form a number of metal contact openings, a first metal contactopening exposing the source structure, a second metal contact openingexposing the drain structure.
 6. The method of claim 5 wherein the firsthard mask layer and the second hard mask layer have different materialcompositions.
 7. The method of claim 5 wherein each mask opening throughthe second hard mask layer has tapered side walls so that a width of amask opening at the top surface of the second hard mask is substantiallygreater than a width of the mask opening at the bottom surface of thesecond hard mask layer.
 8. The method of claim 5 wherein each maskopening through the second hard mask layer has tapered side walls sothat a width of a mask opening at the top surface of the second hardmask is more than twice a width of the mask opening at the bottomsurface of the second hard mask.
 9. The method of claim 5 wherein thethird and fourth hard mask layers are completely removed while the firsthard mask layer is etched.
 10. The method of claim 5 wherein the secondhard mask layer and the third hard mask layer have different materialcompositions.
 11. A method of forming a semiconductor structurecomprising: forming a first hard mask layer that touches and lies overan isolation layer, the isolation layer having a top surface, andtouching and lying over a source structure and a drain structure;forming a second hard mask layer that touches and lies over the firsthard mask layer, the second hard mask structure having a top surface anda bottom surface; forming a third hard mask layer that touches and liesover the second hard mask layer, the third hard mask layer having a topsurface; forming a fourth hard mask layer that touches and lies over thethird hard mask layer, the fourth hard mask layer having a top surface;forming a first patterned photoresist layer that touches and lies overthe fourth hard mask layer; and forming a second patterned photoresistlayer that touches and lies over the fourth hard mask layer, the firstpatterned photoresist layer and the second patterned photoresist layerexposing a number of uncovered regions on the top surface of the fourthhard mask layer.
 12. The method of claim 11 and further comprisingetching the number of uncovered regions on the top surface of the fourthhard mask layer to form a number of trenches that extend through thefourth hard mask layer, each trench exposing the third hard mask layer.13. The method of claim 12 and further comprising: forming a thirdpatterned photoresist layer that touches and lies over the third hardmask layer and the fourth hard mask layer; and forming a fourthpatterned photoresist layer that touches and lies over the third hardmask layer and the fourth hard mask layer, the third patternedphotoresist layer and the fourth patterned photoresist layer exposing anumber of uncovered regions on the top surface of the third hard masklayer.
 14. The method of claim 13 and further comprising etching thethird hard mask layer through the number of uncovered regions on the topsurface of the third hard mask layer, underlying regions of the secondhard mask layer, and underlying regions of the first hard mask layer toform a number of mask openings, the mask openings exposing a number ofuncovered regions on the top surface of the isolation layer.
 15. Themethod of claim 14 and further comprising etching the isolation layerthrough the number of uncovered regions on the top surface of theisolation layer to form a number of metal contact openings, a firstmetal contact opening exposing the source structure, a second metalcontact opening exposing the drain structure.
 16. The method of claim 15wherein the third patterned photoresist layer has a plurality ofspaced-apart strips that each lie substantially orthogonal to thetrenches.
 17. The method of claim 15 wherein the first hard mask layerand the second hard mask layer have different material compositions. 18.The method of claim 15 wherein each mask opening through the second hardmask layer has tapered side walls so that a width of a mask opening atthe top surface of the second hard mask is substantially greater than awidth of the mask opening at the bottom surface of the second hard mask.19. The method of claim 15 wherein the third and fourth hard mask layersare completely removed while the first hard mask layer is etched. 20.The method of claim 15 wherein the second hard mask layer and the thirdhard mask layer have different material compositions.